Slice the flank steak against the grain into paper-thin ribbons and set aside.
Char the halved onion and sliced ginger in a dry pan over medium-high heat until blackened spots appear.
In a medium pot, toast the star anise and cinnamon stick for one minute until fragrant.
Add the beef bone broth, charred onion, ginger, and fish sauce to the pot; simmer gently for fifteen minutes.
While broth simmers, cook rice noodles according to package instructions, then drain and place in a large bowl.
Strain the broth to remove solids and return the liquid to a boil, seasoning with sea salt.
Arrange the raw beef slices over the noodles and pour the boiling broth directly over the meat to cook it instantly.
Top with bean sprouts, scallions, cilantro, basil, jalapeño, and a squeeze of fresh lime.